Lanthanoid aluminate film fabrication method

ABSTRACT

A method of fabricating by co-sputtering deposition a lanthanoid aluminate film with enhanced electrical insulativity owing to suppression of deviation in composition of the film is disclosed. Firstly within a vacuum chamber, hold two separate targets, one of which is made of lanthanoid aluminate (LnAlO 3 ) and the other of which is made of aluminum oxide (Al 2 O 3 ). Then, transport and load a substrate into the vacuum chamber. Next, introduce a chosen sputtering gas into this chamber. Thereafter, perform sputtering of both the targets at a time to thereby form a lanthanoid aluminate film on the substrate surface. This film is well adaptable for use as ultra-thin high dielectric constant (high-k) gate dielectrics in highly miniaturized metal oxide semiconductor (MOS) transistors.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuing application of U.S. application Ser.No. 11/966,304, filed Dec. 28, 2007, now U.S. Pat. No. 8,012,315 thetext of which is incorporated by reference, and is based upon and claimsthe benefit of priority from Japanese Patent Application No.2007-115459, filed on Apr. 25, 2007, the entire content of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to fabrication technologies ofdielectric thin-films for use in highly to integrated semiconductordevices and, more particularly, to a method of forming a lanthanoidaluminate film which is excellent in electrical insulativity, i.e.,dielectricity.

BACKGROUND OF THE INVENTION

As large-scale integrated (LSI) semiconductor circuit devices furtheradvance in miniaturization, a need arises for an ultra-thin dielectricfilm capable of offering enhanced electrical insulation performances. Inprior known LSI chips, a film of silicon oxide (SiO_(x), where thesuffix “x” is usually 2) has been widely used as a multi-purposeinsulative film. However, in near future, it becomes inevitable to useseveral dielectric film materials in a way pursuant to respectiveon-chip circuit element functions in such LSI devices.

Dielectric films for use in the currently available ultralarge-scaleintegrated (ULSI) circuit chips include a gate insulating film of metaloxide semiconductor field effect transistors (MOSFETs), an insulatorfilm between floating and control gates of a memory cell transistor inelectrically erasable programmable nonvolatile semiconductor memorydevices—for example, “Flash” memory of the floating gate type, aninsulator film between charge trapping film and control gates of amemory cell transistor in electrically erasable programmable nonvolatilesemiconductor memory devices—for example, “Flash” memory of the MONOS(metal oxide nitride oxide silicon) type, and a tunneling insulator filmof a memory cell transistor in electrically erasable programmablenonvolatile semiconductor memory devices—for example, “Flash” memory offloating gate type or MONOS type or others. These dielectric films aregenerally required to have high dielectric constant (high-k) and lowleakage current. A promising one of such dielectric films is alanthanoid aluminate film, e.g., a lanthanum aluminate (LaAlO₃) film.

For fabrication of the lanthanoid aluminate film, studies havetraditionally been made to employ pulsed laser deposition (PLD) andmolecular beam epitaxy (MBE) or molecular beam deposition (MBD) methods.These film fabrication methods are inherently research-use processes,which are for preparing small-size samples slowly and carefully atincreased costs. Thus, it does not appear that this approach is welladaptable for use in mass-production of the film. From a viewpoint ofindustrial applications, it is desirable to use sputtering techniques.This can be said because the sputtering is readily applicable to themanufacture of large-size workpieces at low costs while reducingcomplexities in process designs.

A sputtering method of forming a high-k dielectric film, e.g., silicatefilm, for use in ULSI devices is disclosed in JP-A 2003-234471(KOKAI). Aferroelectric film manufacturing method is found in JP-A2003-224123(KOKAI).

SUMMARY OF THE INVENTION

An attempt was made by the inventors as named herein to fabricate bysputter deposition an ultrathin film of lanthanum aluminate (LaAlO₃),which is one of lanthanoid aluminates. In this sputtering, a singletarget was used, which is made of the same material as that of such filmbeing fabricated—i.e., LaAlO₃. However, the lanthanoid aluminate filmthus formed by this single-target sputtering suffers from a problem: thelevel of a leakage current is so high that the film is hardly applicableto ULSI device products.

One main reason of this is as follows. In case the single Al₂O₃ targetis used, electrons behave to attach to oxygen which is large both inelectro-negativity and in electron affinity since the lanthanum is lessin electronegativity. Thus, the generation rate of negative oxygen ionsbecomes higher at or near the surface of the sputtering target. Thisincrease of negative ions induces composition deviation of thelanthanoid aluminate film that is formed on a substrate, resulting in anincrease in leakage current.

More specifically, when negative oxygen ions are generated at highgeneration rate, these ions are accelerated by the voltage potential ofa sheath adjacent to the target in a plasma to have an extra-high energyof several million degrees or above. The negative oxygen ions per se areas less as several milliseconds in half-value period. Thus, mostnegative oxygen ions exhibit dissociation into neutral oxygen particlesand electrons after the acceleration by the sheath voltage. However, theextra-high kinetic energy and momentum of the negative oxygen ions arepassed to and retained by the dissociated neutral oxygens in accordancewith the law of conservation of momentum. This results in generation ofneutral oxygen that is high in energy and in momentum. A high-rate beamof such neutral oxygen and remaining negatively ionized oxygen has itskinetic momentum in a vertical direction with respect to the targetsurface. Upon irradiation of such high-energy/high-momentum particlesonto the substrate with the LaAlO₃ film being presently formed thereon,this can damage not only the LaAlO₃ film deposited thereon per se butalso the substrate beneath the LaAlO₃ film.

The energy of oxygen beam originating negative oxygen ion is as high asseveral million degrees or more; so, atomic motions of lanthanum andaluminum plus oxygen, which are sputtered by plasma particles ofpositive argon ion or others from target and have relatively low energylevel of hundred-thousand degrees, are derived in a LaAlO₃ film that hasexperienced the incoming radiation of the oxygen beams and derivedcascade shower of relatively low energy is particles between severalmillion degrees and hundred-thousand degrees. This would result inselective ejection or “evaporation” of aluminum atoms from the film,which are less in weight than lanthanum atoms. This selective aluminumevaporation is also occurred and facilitated by the fact that aluminumoxides are about 3000° C. in boiling point, which is lower than that oflanthanum oxides—i.e., 4200° C. In this way, the LaAlO₃ film beingformed on the substrate is encountered with unwanted occurrence ofcomposition deviation, resulting in a likewise increase in leakagecurrent of the film.

It is therefore an object of this invention to provide a new andimproved method of making a lanthanoid aluminate film excellent inelectrical insulativity or dielectricity by suppressing the compositiondeviation of the film formed by sputter deposition.

In accordance with one aspect of the invention, a lanthanoid aluminatefilm fabrication method includes, holding within a vacuum chamber afirst target made of lanthanoid aluminate (LnAlO₃) and a second targetmade of aluminum oxide (Al₂O₃), conveying a substrate into the vacuumchamber, introducing a sputtering gas into the vacuum chamber, andsputtering the first and second targets at a time to thereby form alanthanoid aluminate film on the substrate.

According to this invention, it becomes possible to suppress or minimizefilm composition deviation occurring due to execution of a sputteringprocess. This makes it possible to fabricate an ultrathin lanthanoidaluminate film with improved dielectric performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates, in cross-section, a basic structure of a sputteringapparatus for use in the manufacture of a is lanthanoid aluminate filmin accordance with one embodiment of this invention.

FIGS. 2 and 3 are diagrams schematically showing different sectionalviews of a vacuum chamber used in the sputtering apparatus of FIG. 1 forindicating a spatial layout relationship of target and substratesurfaces.

FIG. 4 is a diagram showing model for explanation of the spatial layoutrelation of the target and substrate surfaces.

FIG. 5 and FIG. 6 schematically showing cross-section of target surfaceand substrate surface, respectively.

FIGS. 7 through 10 are graphs showing curves of leakage current oflanthanum aluminate film versus effective oxide thickness(EOT)-converted electric field for various process parameters, includingfilm composition ratio, temperature, and partial pressure of oxygen.

FIGS. 11 and 12 are diagrams depicting different sectional views of thevacuum chamber in the sputter tool of FIG. 1, for explanation of desiredconditions as to rotation of the substrate.

DETAILED DESCRIPTION OF THE INVENTION

In the description of a currently preferred embodiment of thisinvention, the term “lanthanoid aluminate” as used herein refers tocomposite oxide of aluminum and lantanoid element. Examples of thelanthanoid are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium(Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb),dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium(Yb), lutetium (Lu), scandium (Sc) and yttrium (Y).

Additionally in the description below, the leakage current of alanthanoid aluminate film is represented by the magnitude of an electricfield being applied thereto, which in turn is given as an equivalentoxide thickness (EOT)-converted electric field. To determine the EOTelectric field, first convert the thickness of a film under evaluationinto a thickness of silicon oxide (SiO_(x)) film having its dielectricconstant k equivalent thereto. Then, subtract a flat-band voltage fromthe voltage being applied to the lanthanoid aluminate film to therebyobtain a difference voltage. Next, divide this difference voltage by theSiO_(x) film thickness value calculated, and define the resultant valueas the EOT electric field.

A gate insulator film of metal oxide semiconductor field effecttransistors (MOSFETs), an insulative film sandwiched between floatingand control gates of flash memories, and an insulative film of blockinglayer sandwiched between charge accumulation layer and control gate ofmetal oxide nitride oxide semiconductor (MONOS) type flash memories arerequired to be high in dielectric constant k (i.e., high-k) and, at thesame time, low in leakage current. Accordingly, by evaluating a leakagecurrent with the ROT-converted electric field as its index, it becomespossible to properly judge the applicability and usability of such film.In the description of a currently preferred embodiment of thisinvention, the term “composition ratio” means “moler ratio”.

In case a film of lanthanum aluminate, which is one of lanthanoidaluminates, is formed by sputter deposition, an attempt was first madeto perform sputtering from a single or “mono” target that is made oflanthanum aluminate (LaAlO₃). However, with mere use of the LaAlO₃mono-target, it was hardly possible to sufficiently suppress theoccurrence of leakage current flow of the resultant film as statedpreviously.

One reason of the film leakage current increase is that the LaAlO₃ filmdeviates in composition ratio due to the incoming radiation of ahigh-energy beam of neutral oxygen in large part as produced duringsputtering. More specifically, upon irradiation of the high-energyneutral oxygen beam, aluminum is selectively evaporated from the surfaceof LaAlO₃ film whereby the composition of this film is deviated fromthat inherent to LaAlO₃ so that the molar ratio of aluminum (Al) tolanthanoid (Ln) in the film becomes less than 1. This Al/Ln ratioreduction results in an increase in leakage current.

To avoid the composition deviation, an attempt was made to form theLaAlO₃ film by a co-sputtering process using a couple of targets—thatis, a target made of lanthanum oxide (La₂O₃) and a target made ofaluminum oxide (Al₂O₃)—in place of the LaAlO₃ mono-target. However, thisapproach is faced with a problem as to occurrence of target collapse ordecay. It appears that this target decay is resulted from crystalexpansion occurring due to an accelerated change of La₂O₃ to lanthanumhydroxides or lanthanum carbonate hydrates by absorption of a moistureand carbon dioxide (CO₂) in the atmospheric air.

Even though a technique is used for minimizing atmospheric air exposureof the La₂O₃ target, the moisture and CO₂ gas are still absorbable intothe target surface. When using such the target for pre-sputtering orsputtering, carbon components that are originated from the CO₂ gas inthe air are knocked-on deeper into the La₂O₃ target by collision shockreceived from a sputtering gas used, such as argon (Ar) gas. This carbon“invasion” contaminates the lanthanum aluminate (LaAlO₃) filmfabricated.

In view of this point, an attempt was next made to perform chemicalsputtering by use of a lanthanum metal target and an aluminum metaltarget in an oxygen-containing sputter gas atmosphere. This approach,however, suffers from a more serious problem as to further exacerbatedtarget decay. It is likely that this occurred because the lanthanummetal is as high in reactivity as alkali metals.

Furthermore, an attempt was made to perform film fabrication using amono-target made of lanthanum aluminate (LaAlO₃), which has the molarcomposition ratio of lanthanum (La) to aluminum (Al) which was designedin advance so that Al is larger in content than La. Unfortunately it wasrevealed that the stable target can make only from single phasecomposite oxides of lanthanum oxide (La₂O₃)-aluminum oxide (Al₂O₃)system, which exists only when setting a very limited range ofcomposition ratios more precisely, a composition ratio of Al:La=1:1(beta structure), Al:La=1:1 (perovskite structure) or Al:La=7:33, whichis known as “7:33” phase or R phase.

In cases where the target used is with setup of intermediate compositionratios other than these three specific phases, there must occur aproblem similar to that in the case of using the La₂O₃ target statedsupra, i.e., the penalty of target decay due to crystal expansion.

This target decay is due to the presence of residual unreacted lanthanumoxides (La₂O₃). In other words, La₂O₃ other than those used to form theperovskite structure behaves to reside continuously in the target havinga La-increased composition ratio, thus giving rise to the target decay.Alternatively, the target with Al-increased composition ratios decaysdue to the remanence of La₂O₃ that failed to form the perovskitestructure. Another cause of the decay is that lanthanum-doped aluminumoxides (Al(La)₂O₃) have a structure high in content of voids and thusabsorb moisture in the air so that these oxides are unintentionallyconverted to aluminum hydroxides.

The experiment and evaluation results have revealed that a preferableapproach to suppressing the composition deviation of the LaAlO₃ film isto employ a co-sputtering process with the co-use of two separatetargets, one of which is made of LaAlO₃ and the other of which is madeof Al₂O₃.

See FIG. 1, which illustrates schematically a basic configuration of asputtering apparatus suitably used for the lanthanoid aluminate filmfabrication method embodying the invention. The illustrative sputteringtool includes a vacuum chamber 10, a rotation mechanism 14 which isdriven to rotate while at the same time stably supporting thereon asubstrate 12 loaded into the chamber 10, and a heater module 16 forheating the substrate 12. A sputtering cathode electrode 20 is providedto hold and retain a first target 18 and also to apply theretoelectrical power of radio-frequency (RF). Another sputtering cathode 24is for holding a second target 22 and for applying RF power thereto. Agas inlet pipe 26 is provided between the substrate 12 and the targets18 and 22, for feeding a chosen sputtering gas into the interior spaceof vacuum chamber 10. An evacuation pump 28 is provided to exhaustgasses from the inner space of chamber 10.

Within the vacuum chamber 10 one or more magnets (not shown) aredisposed for creating a magnetic field, which is applied to a spacebetween the substrate 12 and the targets 18 and 22. Using thisarrangement enables achievement of RF magnetron sputtering. Targets 18and 22 are spatially laid out so that each is slanted or angled relativeto the substrate 12. This is called the “off-axis” layout.

A film of lanthanum aluminate (LaAlO₃), which is an example of thelanthanoid aluminate, is formed in a way which follows. What is donefirst is to prepare the targets 18 and 22, wherein target 18 is made oflanthanum aluminate (LaAlO₃) whereas target 22 is made of aluminum oxide(Al₂O₃). Then, place the LaAlO₃ target 18 in the vacuum chamber 10 sothat it is stably mounted on the sputtering cathode 20. Similarly setthe Al₂O₃ target 22 on its corresponding is sputter cathode 24. Thesubstrate 12 is also prepared. An example of this substrate is asingle-crystalline silicon (Si) wafer of n-type conductivity, which hasits top surface equivalent to a (100) crystal plane.

Then, apply a cleaning process to the Si wafer 12 by using a dilutedhydrofluoric acid solution, thereby to remove a native-oxide film on thewafer surface. Thereafter, transport the Si wafer 12 and load it intothe vacuum chamber 10. Si wafer 12 is then stably held by the rotationmechanism 14. The surface of LaAlO₃ target 18 to be laid out in such away as to form a prespecified angle θ of ninety (90) degrees relative tothe surface of Si wafer 12 mounted. Similarly, apply position alignmentto Al₂O₃ target 22 so that its surface forms the angle θ of 90 degreeswith the wafer surface.

Referring to FIGS. 2 and 3, one typical layout example is shown forexplanation of the spatial positioning relationship between the Si wafersurface and the surface of one target. FIG. 2 illustrates a sectionalview of the vacuum chamber 10 in a direction normal to Si wafer 12whereas FIG. 3 depicts a sectional view of chamber 10 in a directionparallel to Si wafer 12. It should be noted that only one of the LaAlO₃target 18 and Al₂O₃ target 22—here, target 18 is shown in FIGS. 2-3 forpurposes of convenience in illustration. The same goes with theremaining Al₂O₃ target 22 as far as the surface positioning isconcerned.

As shown in FIG. 2, the Si wafer 12 has a thin round disk-like shapewith a radius “r” (in millimeters). The target/wafer surface angle θ isdefined by a vector T which is at right angles to the target surface andextends in a direction of from the back face to top surface of LaAlO₃target 18 and a vector S that is normal to the wafer surface and extendsin a direction of from the backface to top surface of Si wafer 12. Theangle θ is equal to an angle between these vectors T and S.

As shown in FIG. 4, the LaAlO₃ target 18 is disposed relative to the Siwafer 12 so that a line segment TT′ which is virtually extended from thecenter of mass (or the gravity point) of LaAlO₃ target 18 does not crossa line segment SS′ that is virtually extended vertically from thegravity point of wafer 12; more precisely, line TT′ is in twistedparallel with line SS′. However, line TT′ can cross line SS′ inappropriate case. At this time the minimum distance between lines TT′and SS′—i.e., a distance between points Tc and Sc in FIG. 4 is set to s1(mm). s1 (mm) can be 0 (mm) in the appropriate case. This distancesetting is similarly applicable to the other Al₂O₃ target 22: let itsminimum distance be s2 (mm).

The target may have various shapes, for example—a plate-like, acylinder-like, a sphere-like, a waved-plate-like, a concentricwaved-plate-like, a fan-like, a sphere-core-like, a rectangularparallelepiped-like, a disk-like. If the surface of LaAlO₃ target 18 isnot a completely flat plane, a flat approximate surface of the targetobtained by the method of least squares is used. Data points for themethod are selected from the actual surface of the target evenly and asmany as possible for better approximation. An example is shown in FIG.5, wherein the actual surface of the target is waved irregularly and theapproximate surface of the target which is a flat plane obtained fromthe model is indicated by a thick line. The same goes with the Al₂O₃target 22.

A substrate, e.g. Si wafer, may also have various shapes, for example—aplate-like, a cylinder-like, a sphere-like, a waved-plate-like, aconcentric waved-plate-like, a sphere-core-like, a rectangularparallelepiped-like, a disk-like. If the surface of Si wafer 12 is notcompletely flat, use a surface model that approximates the actual wafersurface. Similar model used to define the approximate surface of thetarget can be applicable for defining the approximate surface of thesubstrate. An example is shown in FIG. 6, wherein the actual surface ofthe substrate is of a hemispherical shape and the approximate surface ofthe substrate which is a flat plane obtained from the model is indicatedby a thick line.

After having disposed the loaded Si wafer 12 to satisfy the above-statedpositional relationship for the LaAlO₃ target 18 and Al₂O₃ target 22,apply sputtering to these targets at a time to thereby deposit a LaAlO₃film on the surface of wafer 12. Typical process conditions at this filmfabrication step are as follows:

Vacuum of chamber: 1×10⁻⁸ Pa

Wafer radius: 150 mm

Wafer/target angle θ: 90 degrees

Minimum LaAlO₃ target-wafer distance s1: 100 mm

Minimum Al₂O₃ target-wafer distance s2: 100 mm

LaAlO₃ target radius R1: 32 mm

Al₂O₃ target radius R2: 27.5 mm

Sputter gas: mixture of Ar and O₂ gases

Total gas pressure: 1.0 Pa

Partial O₂ pressure: 1.0×10³ Pa

Wafer temperature: 600° C.

RF power of LaAlO₃ target: 50 W

RF power of Al₂O₃ target: 33 W

Film thickness: 22 nm

Film forming rate: 0.025 nm/sec.

Film growth time: 880 sec.

Wafer spin rate: 0.3 turns/sec.

Wafer rotation speed: 0 turns/sec.

In the LaAlO₃ film forming process under the exemplary conditions statedabove, prior to introducing the sputter gas into the vacuum chamber 10,the vacuum pump 28 of FIG. 1 is driven to perform evacuation so that theinside space of chamber 10 is kept at the above-noted level of a vacuum.Thereafter, the sputter gas of Ar is fed thereinto from the gas inletpipe 26.

Then, activate the sputter cathodes 20 and 24 to apply n RE power withthe values stated supra to the LaAlO₃ target 18 and Al₂O₃ target 22.While appropriately controlling the ratio of RF power to cathode 20versus RF power to cathode 24, adjust the molar ratio of Al to La of thelanthanum aluminate (LaAlO₃) film being formed directly on Si wafer 12.In this way, the LaAlO₃ film obtained has its Al/La ratio of 1.00±0.02as an example, wherein the value 0.02 is a practically allowablemeasurement error.

Additionally, an oxygen (O₂) gas is also introduced into the vacuumchamber 10 by another gas inlet pipe (not shown in FIG. 1); then,control is provided to ensure that the partial pressure of O₂ gas is setat the aimed value stated above. The feed rates of Ar and O₂ gases andthe gas exhaust amount of vacuum pump 28 are adjusted to thereby controlthe total pressure of the sputtering gas (here, also including thepartial pressure of O₂) so that this is set to the above-identifiedvalue. Si wafer 12 is heated by heater 16 so that it is kept at atemperature of the above-noted value. Control of the spin rate is doneby the rotation mechanism 14 shown in FIG. 1.

See FIG. 7, which is a graph showing leakage characteristics of thelanthanum aluminate (LaAlO₃) film thus formed under the processconditions stated supra—that is, a curve of leakage current per unitarea versus EOT-converted electric field. For comparison purposes, thecharacteristic curve of a LaAlO₃ film is also shown, which was formed byprior known method with the aid of a single or “stand-alone” LaAlO₃target at room temperature. This comparative LaAlO₃ film sample is 0.76in molar Al/La ratio. As apparent from is the graph of FIG. 7, theLaAlO₃ that was formed by the co-sputtering method embodying theinvention is noticeably lowered in leakage current and thus offersenhanced electrical insulation performance.

One factor that contributes to reduction of the leakage current lies insuppressing deviation of Al/La composition of the LaAlO₃ filmfabricated. To verify this, several film samples of different molarAl/La composition ratios were prepared for measurement of leakagecurrent characteristics thereof. Measurement results are graphicallyshown in FIG. 8 namely, leakage characteristics of samples with Al/Lacomposition ratios of 1.00 and 1.19 which were fabricated by theembodiment cosputtering method along with the leak characteristics of acomparative sample with its Al/La composition ratio of 0.76 which wasformed using the mono-target, which is similar to that of FIG. 7. Thesesamples were set at the same wafer temperature during sputterdeposition. The Al/La ratio of every sample was adjusted by control ofthe sputter cathode RF power ratio.

As apparent from viewing the graph of FIG. 8, by increasing themono-target's molar Al/La composition ratio by use of the embodimentcosputtering method from 0.76 to 1.0 and to 1.19, the resulting LaAlO₃film was appreciably suppressed in leakage current flowable therein.Note however that when setting the molar Al/La ratio at a value greaterthan 1, e.g., 1.19, the leakage current suppressibility was decreased bylittle. From these experiment results, it has been made sure that inorder to successfully reduce the occurrence of leakage current with torespect to the applied voltage, it is preferable to set the LaAlO₃film's Al/La ratio to fall within the range of from about 0.98 to about1.19. More preferably, the ratio is set to range from 0.98 to 1.02. Withthis ratio setup, it is possible to further reduce the leakage currentrelative to is the voltage applied.

Letting the Al/La ratio come close to 1 in this way results in avoidanceof the composition deviation of LaAlO₃, thereby making it possible toform the intended perovskite structure with increased stability in filmquality. Accordingly, the dielectric constant k increases in value. Thisleads to reduction of a leakage current otherwise occurring due toimperfectness of crystallinity. Thus the leakage current with theEOT-converted electric field as its index or “barometer” is suppressed.In addition, while an increase in Al's composition ratio tends to causethe LaAlO₃ film to expand in its band gap, it is considered that thisband gap expansion also contributes to the suppression of the leakagecurrent.

Although in this embodiment the sputter deposition is performed by theoff-axis layout scheme with the Si wafer 12 being disposed so that itssurface is angled at 90 degrees with respect to each of the LaAlO₃target 18 and Al₂O₃ target 22, this wafer may alternatively be laid outso that its surface opposes vis-a-vis each target. In other words, eachtarget 18, 22 is at an angle θ of 180 degrees relative to Si wafer 12when the need arises. In this case, however, wafer can accompany therisk of incoming radiation of an increased amount of high-energy neutraloxygen beam. This irradiation beam increase results in acceleration ofselective evaporation of aluminum, thereby making it difficult tocontrol the molar Al/La composition ratio. Additionally the irradiationof high-energy neutral oxygen beam can result in creation of defects inthe LaAlO₃ film fabricated, resulting in an increase in leakage current.

By taking these risks into consideration, it is desirable to employ theoff-axis layout with the both surfaces of LaAlO₃ and Al₂O₃ targets 18and 22 being tilted with respect to the Si wafer 12. Preferably thewafer-target angle θ is set to range from 70 to 110 degrees. This anglesetting is preferable for the reasons which follow. If this angle θbecomes larger than 110 degrees, selective evaporation of aluminum dueto high-energy beam of neutral oxygen being irradiated onto Si wafer 12increases resulting in unwanted creation of defects in the LaAlO₃ filmformed. On the contrary, if the angle θ is less than 70 degrees, thegrowth rate the LaAlO₃ film on wafer decreases, resulting in an increasein manufacturing costs.

In the film forming conditions stated previously, the substrate or wafertemperature during sputter deposition is set at 600° C. This is becauseletting this temperature be higher than room temperature leads tosuppression of leakage current as shown in FIG. 9. More generically, thewafer temperature may be set to range from 300 to 650° C. The reason ofthis is as follows. If the wafer temperature goes beyond 650° C.,residual materials on Si wafer 12 are thermally driven into the LaAlO₃film, resulting in changes in composition and structure of the film. Ifthe wafer temperature becomes less than 300° C., the short-distanceorder of atoms fails to reach the aimed thermal motion energy fortransition into the minimal energy state of perovskite structure,resulting in establishment of non-perovskite short-distance order, whichcauses the dielectric constant to become lower than that of theperovskite. To suppress lowering of the dielectric constant, setting thewafer temperature equal to or higher than 500° C. is more preferable.Additionally the short-distance order of atoms in LaAlO₃ film is of thecoherence of about 2 nm at most, so whether the coherence is present orabsent is not determinable by standard x-ray diffraction methods.However, by using extended x-ray absorption fine structure (EXAFS)methods to observe the spectrum near an absorption end, it becomespossible to affirm not only a difference in short-distance atom orderbut also such difference due to a difference in wafer temperature.

While in the film-forming process conditions the partial pressure ofoxygen in the sputtering gas is set at 1.0×10⁻³ Pa, this pressure may bemore generically designed to range from 1.6×10⁻⁶ to 3.0−10⁻³ Pa. Thereason of this is as follows. In case the oxygen's partial pressure isset at 0, the LaAlO₃ film formed can increase in leakage currentespecially when the wafer is at high temperatures. From a viewpoint ofapparatus control, it is usually difficult to control an oxygen partialpressure of less than 1.6×10⁻⁶ Pa with sputtering deposition. Inaddition, as shown in FIG. 10, the leakage current of LaAlO₃ film canincrease when this pressure becomes higher than 1.0×10⁻³ Pa also.Accordingly, if the oxygen partial pressure becomes higher than 3.0×10⁻³Pa then the above-noted advantage is lost as to the leakage currentsuppressibility of the cosputtering method of this embodiment over priorknown mono-sputtering approaches. Additionally a curve of leakagecurrent versus EOT electric field with the oxygen partial pressure setat 3.0×10⁻³ Pa shown in FIG. 10 was obtained while changing this partialpressure only.

The increase in leakage current of LaAlO₃ film due to an increase inoxygen partial pressure is considered to have roots in the ratio ofactive species of the oxygen. More specifically, the oxygen receivesmutual interaction of a plasma during film fabrication and thus existsin the form of molecules or ionized particles, such as O₂, O₂ ⁺, O₂ ⁺,O⁺, O₂ ²⁻, O(¹D), and O(³P). Negative ions, such as O₂ ⁻, O⁻ and O₂ ²⁻,have an increased momentum through high-speed acceleration by thesheath, which leads to the risk that defects are induced in the LaAlO₃film on Si wafer as stated previously.

Regarding other particles such as O₃, O(¹D), etc., these are extra-highin internal energy so that they are active and high in efficiency ofon-wafer LaAlO₃ film oxidation. On the contrary, the particles are keptlow in momentum as far as they are not originated from O⁻, O₂ ⁻, nor O₂²⁻, and are thus relatively less in risk of inducing defects in LaAlO₃film. Therefore, appropriately setting the oxygen partial pressurecontributes to the reduction of leakage current. However, when theoxygen partial pressure in the sputtering gas increases, active oxygenparticles are made inactive by the interaction: O(³D)+O₂→O(³P)+O₂,O₃→O(¹D)+O₂→O(³P)+O₂. This results in a decrease in the ratio of O₃,O(¹D) or else, i.e., the particle species preferable for sputterdeposition of the LaAlO₃ film. Thus, it is considered that when theoxygen partial pressure becomes higher also, the LaAlO₃ film formedincreases in leakage current. Note that the decrease in ratio of O₃,O(¹D) tends to occur when the oxygen partial pressure is set to 1.0×10⁻³Pa or above.

In view of the phenomena stated above, in the LaAlO₃ film fabricationprocess of this embodiment, it is preferable from a viewpoint of leakagecurrent reduction that the active oxygen, such as O₃ or O(¹D), be feddirectly from an active oxygen generator device to a location near theSi wafer 12 in the sputtering gas atmosphere.

Although in the process conditions listed the LaAlO₃ film growth rate isset at 0.025 nm/sec., it is preferable in a more generic sense to set itto fall within a range of from 0.0003 to 0.04 nm/sec. One reason of thisis that setting the rate to less than 0.0003 nm/sec. makes it difficultto stably retain the plasma at or near the targets. To stabilize theplasma, use of a massive or “giant” vacuum chamber is required,resulting in a cost increase of the sputtering apparatus. Another reasonis that the setup at such slow rate causes the fabrication process todecrease in efficiency. On the contrary, if the film forming rate isexceeds the upper limit value of 0.04 nm/sec., electrical power requiredincreases, resulting in a decrease in target use efficiency. This leadsto an increase in production costs.

In the above-stated process conditions the wafer radius r is set to 150mm whereas the minimum distance s1, s2 of each of the LaAlO₃ and Al₂O₃targets 18 and 22 relative to Si wafer 12 is set at 100 mm. Moregenerically, these values r and s1−s2 may be determined to satisfy:r/2≦s1≦r+30, and r/2≦s2≦r+30. With this value settings, it is possibleto improve the inplane uniformity of LaAlO₃ film thickness.

The reason for this is as follows. If the minimum target-wafer distances1 or s2 becomes larger than the value r+30, the LaAlO₃ film formed onSi wafer 12 can become thicker unintentionally at portions near thewafer edge under ordinary sputter deposition conditions. Adversely, ifthe distance s1, s2 becomes less than the value r/2, the on-wafer LaAlO₃film usually becomes thicker at portions near the wafer center underordinary sputtering conditions.

From a viewpoint of the film thickness uniformity, it is preferable todetermine the values r, s1 and s2 to satisfy the above-stated relations;however, from viewpoints of increased sputter condition adjustability,apparatus malfunction reduction by structure simplification, and costreduction by chamber downsizing and easiness of system design, it isrecommendable to set the distance values s1 and s2 to satisfy: s1=s2=0(mm).

In the above-listed process conditions the total pressure of thesputtering gas is set at 1.0 Pa. More generically this pressure may beset to range from 0.1 to 2.0 Pa. The reason of this is as follows. Ifthe total gas pressure is less than 0.1 Pa, a plasma created can becomeless in stability with the use of standard charge-coupled sputtercathodes. If this pressure exceeds 2.0 Pa, is sputtered particles can beshielded or “blocked” by the sputter gas, resulting in a decrease infilm forming rate. Another risk is that the LaAlO₃ film formeddeteriorates in leakage characteristics due to unwanted acceptance ofimpurities in the sputter gas.

In the process conditions listed, the degree of vacuum of chamber 10prior to introduction of the sputtering gas is set at 1×10⁻⁸ Pa. Moregenerically this vacuum may be set to 8×10⁻⁶ Pa or below—preferably, at2×10⁻⁸ Pa or less. The reason of this is as follows. If the chambervacuum becomes greater than 8×10⁻⁶ Pa, impurities can be mixed into theLaAlO₃ film at practical film-forming rates due to a deficient level ofvacuum, resulting in a decrease in film dielectricity. As far as thevacuum is at 2×10⁻⁸ Pa or less, such impurity mixture is effectivelyprevented, resulting in further improvement of dielectriccharacteristics of the LaAlO₃ film.

Although in the above-stated film forming conditions the spin rate of Siwafer 12 is set at 0.3 turns/sec., such wafer spinning may beeliminatable when the need arises. Note however that spinning wafer 12makes it expectable to improve the inplane uniformity of LaAlO₃ film.Preferably, the wafer spin rate, Vs, may be set to satisfy: 1/t≦Vs≦1082,where t is the film growth time (sec.), that is, the time between abeginning of growth of the film on the substrate and an end of thegrowth of the film. It is also permissible to set the revolution or“go-around” speed Vo (turns/sec.) of the wafer to satisfy: 1/t≦Vo≦1082.More preferably, let Vs satisfy 3/r≦Vs≦10; similarly, let Vo satisfy3/t≦Vo≦10.

The reason of this is as follows. If the wafer spin rate Vs and waferrevolution rate Vo are each less than 1/t, the wafer obviously fails tocomplete one turn within the film-forming time period. Thus, the LaAlO₃film can become deficient in improvement of inplane uniformity. Whenwafer 12 is driven to perform three turns at least, the film isappreciably improved in inplane uniformity. Alternatively, if the rateVs, Vo is in excess of 1082 (turns/sec.), physical or mechanicalvibrations become stronger at air-contact portions of a rotatable shaftfor transmission of a rotation torque to the vacuum chamber 10, therebymaking it difficult to perform the intended film fabrication. Settingeach rate Vs, Vo at 10 enables achievement of smooth wafer rotation,thus making it possible to perform the film fabrication with enhancedstability.

A detailed explanation will now be given of desired conditions as to thewafer-spin/rotation drive with reference to FIGS. 11 and 12 below. FIG.11 is a pictorial representation of the vacuum chamber 10, which showsits cross-section at right angles to Si wafer 12 whereas FIG. 12 depictsa sectional view of chamber 10 in parallel to the wafer. Even where morethan two wafer rotation shafts are provided, a locus or “orbit” of therotating wafer's gravity point per se becomes a circle in shape. Thiswill be called the locus circle. Define a straight line segmentextending from a central point of this locus circle in a directionnormal to a plane containing this circle, which line will be regarded asan overall or “global” rotation axis of Si wafer 12. Let the center ofthe locus circle be a global center C of wafer 12.

As shown in FIGS. 11-12, the x-axis and y-axis of an orthogonalcoordinate system are defined with the global rotation center of Siwafer 12 being as the original point thereof. The x-axis is a straightline which is uniquely defined at a position whereat the wafer surfacecrosses a flat plane containing therein both the global rotation axis ofwafer 12 and the gravity point of LaAlO₃ target 18. This x-axis extendsin a direction along which a distance to the is target 18's gravitypoint becomes less in value. The y-axis is a straight line extending ina direction along which the distance between Si wafer 12 and the targetgravity point of target 18 in the global rotation axis direction becomessmaller.

With this x- and y-axes setting, the closer to the target, the largerthe x/y-axis coordinate value (mm). In this x-y coordinate system, thegravity point of the LaAlO₃ target 18 is represented by a coordinatepoint (xt1,yt1), with its radius given by R1 (mm). Regarding the Al₂O₃target 22, its gravity point is represented as (xt2,yt2); let its radiusbe R2. Let the radius of Si wafer 12 be r. At this time, LaAlO₃ target18 is laid out to satisfy: rxt1 and R1+10≦yt1. For Al₂ ^(O) ₃ target 22,let it satisfy: r≦xt2 and R2+10≦yt2. This target positioning design ispreferable for the reason which follows: if the value xt1 is less thanr, the LaAlO₃ film fails to be uniformly formed on the wafer; if yt1 isless than R1+10 then LaAlO₃ film can “overflow” and is possibly formedon the backface of wafer 12 also. The same goes with the values xt2, yt2and R2 of the other target 22.

In the above-listed process conditions the Ar gas is used as thesputtering gas. Other kinds of gases are alternatively employable, eachof which contains a noble gas as its main component. Examples of thenoble gas include but not limited to a krypton (Kr) gas, xenon (Xe) gas,neon (Ne) gas, and helium (He) gas. Note however that the use of Argas-based sputtering gas is preferable from a viewpoint of costreduction in industrial applications.

Although in the illustrative embodiment the lanthanum aluminate (LaAlO₃)is used as one example of the lanthanoid aluminate film material, thelanthanum (La) may be replaced by any other element or elements asselected from the lanthanoid (Ln) group including cerium (Ce),praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), isgadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), scandium (Sc) andyttrium (Y). In this case also, similar effects and advantages areobtainable.

Additionally, every lanthanoid aluminate (LnAlO₃) film fabricated hasthe perovskite structure. As aluminum (Al) is less in weight than Lnelements, Al is readily evaporated from the LnALO₃ film. The selectiveAl evaporation is also facilitated by the fact that the boiling point ofaluminum oxides is at 3000° C., which is lower than that of lanthanoidoxides. Accordingly, in the case of a mono-target made of LnAlO₃ beingused, film composition deviation can take place as in the case oflanthanum (La). This composition deviation is curable by thecosputtering method using the lanthanoid aluminate (LnAlO₃) target andaluminum oxide (Al₂O₃) target at a time. Though sputter yields oflanthanoid increases from the minimum atomic number of lanthanum (La) tomaximum atomic number of lutetium (Lu), this fact decreases Al/Ln ratiowith sputtering from mono-target of LnAlO₃ furthermore. Thereforecosputtering is more effective in these cases.

The composition is adjustable by varying the ratio of RF power appliedto LaAlO₃ target 18 and RF power to Al₂O₃ target 22. For example, forseveral samples prepared while changing the target-applied RF powerratio, analyze composition changes of these samples by inductivelycoupled plasma (ICP) spectrometry or like techniques, thereby toquantitatively determine an optimal RF power ratio. Other film sputterdeposition process conditions are determinable depending on the physicalor chemical nature of the lanthanoid element used. Thus, in the case oflanthanoid aluminate (LnAlO₃) films other than the LaAlO₃ film also,similar process conditions to those of LaAlO₃ film fabrication areemployable. This can be said because lanthanoid elements are similar inchemical properties to one another.

Although the invention has been disclosed and illustrated with referenceto a particular embodiment, the principles involved are susceptible foruse in numerous other embodiments which will be apparent to personsskilled in the art. While in the description above those parts of thelanthanoid aluminate film fabrication method and apparatus which aredeemed not to be directly related to the principal concept of theinvention are not specifically described, it would readily occur toskilled persons that these may be arranged and designed by selective useof currently known elements of such method and apparatus in a known way.The invention is, therefore, to be limited only as indicated by thescope of the appended claims.

What is claimed is:
 1. A method of fabricating lanthanum aluminate film,the method comprising: holding within a vacuum chamber a first targetcomprising LaAlO₃ or a compound comprising aluminum, lanthanum andoxygen having a ratio of Al:La=7:33 and a second target comprising Al₂O₃or a compound comprising aluminum, lanthanum and oxygen having a ratioof Al:La=11:1; conveying a substrate into the vacuum chamber;introducing a sputtering gas into the vacuum chamber; and sputtering thefirst and second targets to thereby form a lanthanum aluminate film onthe substrate.
 2. The method of claim 1, wherein the lanthanum aluminatefilm has a molar ratio of aluminum (Al) to lanthanum (La) falling withina range of from 0.98 to 1.19.
 3. The method of claim 1, wherein a molarratio of Al to La in the lanthanum aluminate film ranges from 0.98 to1.02.
 4. The method of claim 1, wherein the first and second targets areplaced such that an angle of a surface of the substrate formed with asurface of the each target ranges from 70 degrees to 110 degrees.
 5. Themethod of claim 1, wherein, during the sputtering, the substrate is setat a temperature ranging from about 300° C. to about 650° C.
 6. Themethod of claim 1, wherein the sputtering gas comprises therein oxygenhaving a partial pressure ranging from 1.6×10⁻⁶ Pa to 3.0×10⁻³ Pa. 7.The method of claim 1, wherein the lanthanum aluminate film is formed ata rate ranging from 0.0003 nanometers per second (nm/sec) to 0.04nm/sec.
 8. The method of claim 1, wherein the first and second targetsare laid out to satisfy a relation of r/2≦s1≦r+30 and r/2≦s2≦r+30,wherein r (unit is millimeters or “mm”) is a radius of the substrate, s1(in mm) is a minimum distance between a line segment passing through agravity point of the substrate and extending at right angles to asurface of the substrate and a line segment passing through a gravitypoint of the first target and extending at right angles to a surface ofthe first target, and s2 (mm) is a minimum distance between the linesegment passing through the gravity point of the substrate and extendingat right angles to the surface of the substrate and a line segmentpassing through a gravity point of the second target and extending atright angles to a surface of the second target.
 9. The method of claim1, wherein the sputtering gas has a total pressure ranging from 0.1 Pato 2.0 Pa.
 10. The method of claim 1, wherein, prior to introducing ofthe sputtering gas, the vacuum chamber has a degree of vacuum of lessthan or equal to 2×10⁻⁸ Pa.
 11. The method of claim 1, furthercomprising: rotating the substrate during the sputtering.
 12. The methodof claim 11, wherein the substrate is rotated while satisfying at leastone selected from the group consisting of a relation of 1/t≦Vs≦082 and arelation of 1/t≦Vo≦1082, wherein Vs is a spin speed of the substrate(turns per second), Vo is an orbital speed of the substrate (turns persecond), and t is a growth time (seconds) for fabrication of thelanthanum aluminate film.
 13. The method of claim 1, further comprising:supplying at least one selected from the group consisting of O₃ andO(¹D) into the sputtering gas from an active oxygen generating device.14. The method of claim 1, wherein the sputtering gas comprises an argon(Ar) gas as its main component.
 15. The method of claim 1, wherein aratio that is RF power of the first target/RF power of the secondtarget, is about 50/33.
 16. The method of claim 1, wherein a thicknessof the lanthanum aluminate film is about 22 nm.
 17. The method of claim1, wherein the sputtering gas comprises a noble gas selected from thegroup consisting of argon (Ar) gas, krypton (Kr) gas, xenon (Xe) gas,neon (Ne) gas, and helium (He) gas, as its main component.
 18. Themethod of claim 1, wherein the substrate, the first target, and thesecond target are laid out to satisfy a relation of r≦xt1, R1+10≦yt1,r≦xt2 and R2+10≦yt2, wherein x-axis and y-axis of an orthogonalcoordinate system are defined with a global rotation center of thesubstrate being as the original point thereof, wherein the x-axis is astraight line which is uniquely defined at a position where thesubstrate surface crosses a flat plane containing therein both theglobal rotation axis of the substrate and the gravity point of eitherthe first target or the second target, wherein the y-axis is a straightline extending in a direction along the global rotation axis, wherein ris a radius of the substrate, xt1 and yt1 are x-coordinate andy-coordinate of the gravity point of the first target, respectively, xt2and yt2 are x-coordinate and y-coordinate of the gravity point of thesecond target, respectively, and R1 and R2 are radius of the firsttarget and the second target respectively.
 19. The method of claim 1,wherein said first target comprises a compound comprising aluminum,lanthanum and oxygen having a ratio of Al:La=7:33 and said second targetcomprises a compound comprising aluminum, lanthanum and oxygen having aratio of Al:La=11:1.